Gas off-take system



March 1967 A. M. FRENDBERG ETAL 3,310,035

GAS OFF TAKE SYSTEM Filed Sept. 28, 1965 s Sheets-Sheet 1 FIG. 1

. INVENTORS Ar1hur M. Frendberg BY Richard Siegfried ATTORNEY March 21,1967 A. M. FRENDBERG ETAL GAS OFF-TAKE SYSTEM 3 Sheets-Sheet 2 FiledSept. 28, 1965 MaI'Ch 21, 1967 A. M. FRENDBERG ETAL 36 GAS OFF-TAKESYSTEM Filed Sept. 28, 1965 j 3 Sheets-sheet s (LIONVECTJON 4.

4 I l PASS I 4 I I TO AIR HEATER United States Patent poration of NewJersey Filed Sept. 28, 1965, Ser. No. 490,813 8 Claims. (Cl. 1221) Thisinvention relates generally to a gas off-take system, i.e. apparatus forwithdrawing a portion of gas from a main gas stream, and moreparticularly to such a system as used in conjunction with a vaporgenerating unit employing the recirculation of cooled gaseous combustionproducts to the furnace for purposes of vapor temperature control.

In the design and operation of modern high capacity steam generatingplants, the recirculation of gaseous combustion products is commonlyused as a means of altering the heat absorption pattern Within the steamgenerator to effect substantially constant outlet steam temperatureconditions over a wide load range. The gas to be recirculated iswithdrawn from a relatively cool region (usually after the economizer)of the steam generator and is reintroduced to the furnace by means of asuitable gas recirculation system including a fan and connectingductwork. Where coal or other high-ash fuel is burned in the furnace,the gaseous combustion products leaving the furnace contain significantquantities of particulate matter which, it carried over into the gasrecirculation system, may cause serious erosion problems in the gasrecirculation fan. Consequently, it is common practice to place a dustcollector in the gas recirculating system upstream of the gasrecirculation fan; however, such collectors are expensive and occupy aconsiderable amount of prime space in the immediate vicinity of thesteam generator.

Accordingly, it is an object of the present invention to provide animproved gas off-take arrangement for withdrawing from the steamgenerator outlet the gas to be recirculated to the furnace. It is astill further object that this arrangement be effective for purposes ofexcluding from the gas recirculating system at least a substantialportion of the solid particulate matter carried by the main flue gasstream of the steam generator. Still further objects of the presentinvention are that the gas oil-take apparatus be arranged to affordstructural rigidity to the duct from which the gas is being withdrawn,and to provide for the uniform withdrawal of gas from substantially theentire cross-sectional flow area of the main gas stream.

According to the present invention, these and other objects may beattained in a vapor generating unit having a furnace chamber whereinash-bearing carbonaceous fuel is burned to produce solids-laden hightemperature heating gases. An upright heating gas pass, communicating atits upper end with the furnace discharge, has arranged therein tubularheat exchange elements including an economizer. An air heater isconventionally arranged adjacent to and connects with the uprightconvection gas pass outlet for the flow of flue gas therefrom by a mainduct which preferably includes a substantially right angle bend. A gasoff-take system is arranged within the duct at the bend portion thereof,and includes a gas collecting and conveying duct interconnecting themain duct and the furnace chamber, and having interposed therein a gasrecirculation fan. Within the main duct, and s aced at regular intervalsthereacross, are a plurality of off-take channel elements, each of whichis connected to opposite sides of the duct to afford structural rigiditythereto. Each channel element is in fluid flow communication with anintake manifold of the gas collecting duct, and

has formed therein inlet opening rneans disposed Within the main duct ina plane having a substantial horizontal component and beingsubstantially perpendicular to the flow of the solids-laden gas in themain duct, whereby gas flowing from the main duct through the gascollecting duct is required to take a substantially turn to enter thechannel members.

For a better understanding of the invention, its operating advantagesand specific objects attained by its use, reference should be had to thefollowing description which refers to the accompanying drawing in which:

FIG. 1 is a diagrammatic sectional side elevation of a vapor generatorhaving a gas recirculation system wherein one embodiment of the gasoff-take system of the present invention is employed; FIG. 2 is anenlarged plan view, having portions thereof broken away, taken alongline 22 of FIG. 1; FIG. 3 is a diagrammatic sectional side elevationtaken along line 33 of FIG. 4 of an alternate gas off-take systemaccording to the present invention; and FIG. 4 is a partial sectionalplan view taken along line 44 of FIG. 3.

Referring to FIG. 1 of the drawings, the main portions of the unit shownare an air heater 10, an upright furnace 12, a horizontal gas pass 14and an upright convection gas pass 16. The boundary Walls of the furnace12 and gas passes 14 and 16 are lined, in the conventional manner, withtubes through which fluid to be heated is passed for the absorption ofheat by radiation and/or convection. Ambient temperature combustion airis supplied by a forced draft fan (not shown) via inlet duct 18 to theair heater 10, wherein the combustion air is passed in indirect heatexchange relation with combustion flue gas. The heated air passes viaoutlet duct 20 to the windbox 22 for distribution to the fuel burningequipment which includes a plurality of cyclone furnaces 24 wherein fuel(usually crushed coal) is thoroughly mixed with the air and burned toproduce high temperature heating gases which are discharged into thefurnace 12. The heating gasses pass upwardly through the furnace, thenlaterally through the horizontal gas pass 14 containing vapor heatingsections 26, and then downwardly through theconvection gas pass 16 whichhas disposed therein additional vapor heating sections 28 and aneconomizer 30. A major portion of the heating gases then passes, in aU-shaped flow pattern, through the air heater 10 prior to beingdischarged to the atmosphere via duct 32.

For purposes of controlling steam temperature, a portion of thepartially cooled heating gases may be withdrawn from the main gas streamat a location between the economizer 50 and the air heater 10, andrecirculated to the furnace 12. Withdrawal of the gas to be recirculatedis accomplished in a gas off-take system designated generally by thenumeral 50 in FIG. 1, which system will be described in greater detailhereinafter. The gas withdrawn from the main stream is collected in thehorizontally disposed cross duct or intake manifold 51 which connects ateach of its ends (see FIG. 2) with a fan inlet duct 52 through which therecirculating gas is conveyed to a gas recirculating fan 55, orpreferably to a pair of such fans operating in parallel. The fan (orfans) 55 provides the necessary energy to overcome the pressuredifferential between the point of withdrawal of the recirculated gasesfrom the main gas stream and the furnace, including the pressure lossthrough the gas recirculating system. The recirculated gas passes fromthe fan 55 via fan outlet duct 53 to the lower recirculated gas plenum56, from whence it is introduced into the fur nace 12 by way ofrecirculation gas ports 56A. Recirculated gas may also be deliveredthrough the upwardly extending conduit 57 to the upper recirculated gasplenum 58 for introduction into the upper region of the furnace 12through gas tempering ports 58A. It should be recognized that suitabledampers (not shown) may be pro vided in the gas recirculation systemducts to afford means for controlling the flow of recirculated gas andits apportionment to the upper and lower ports 58A and 56A.

Referring now particularly to the gas off-take system of FIGS. 1 and 2,the bottom of the convection pass 16 is formed with an ash collectinghopper 17, with suit able provisions for in-service clean out, and anupright convection pass outlet 16A. It should be noted that the airheater is significantly wider than the convection pass 16 (see FIG. 2)as is ofttimes the situation, depending on air heater arrangement andheating surface requirements. A sharply flaring, horizontally disposedtransition duct section 61) interconnects the convection pass outlet 16Aand the air heater inlet duct 19. Expansion joints 61A and 61B areprovided in the transition duct section 61) to accommodate differentialthermal expansion between the air heater 1t and the walls defining theconvection gas pass 16.

Economy of space and cost dictate that the transition duct section 60 beas short as practicable. However, the resulting sharp divergence of theduct section 60 tends to cause an intolerable maldistribution of fluegas at the air heater inlet, with the gas flow being concentrated in thelateral center portion of the air heater. To correct thismaldistribution, upright directional vanes 62, extending throughout theheight of the transition duct section, are arranged at regularly spacedintervals thereacross, the vanes 62 being symmetrically oriented withrespect to each other to provide equal angles of divergence betweenadjacent vanes 62.

Oriented in planes coextensive with the planes of the directional vanes62 are a plurality of gas off-take boxes or channel elements 65 disposeddirectly above the first heating gas pass of the air heater, the boxesbeing arranged in the zone Where the flue gases are constrained toexecute a right angle downward turn as indicated by the flow directionarrow 64. Each off-take box 65 includes a pair of spaced substantiallyparallel side plates 66 joined at their upstream edges by a vertical endC10- sure 67 which is preferably tapered to a relatively sharp edge toavoid excessive pressure drop and gas flow disturbance. The ends of theside plates 66 abut the plate which forms a part of the air heater inlet19, so that the box ends opposite the end closure 67 are also closed.Each of the off-take boxes is open at its upper and lower ends only toafford a flow channel between the main gas stream and the intakemanifold 51 in which the withdrawn gases are collected for conveyance tothe gas recirculating fan 55. The spaces between adjacent boxes 65 areclosed at the upper ends of the boxes by plates 68.

In operation, the solids-laden main gas stream passes downwardly betweenthe off-take boxes 65 upon making the right angle turn from thehorizontally disposed transition duct section 66 to the air heater inletduct 19. Uniform distribution of the gases to the air heater inlet duct19 is enhanced because of the above described orientation of thedirectional vanes 62 and the boxes 65. As the main gas stream passes thelower edge of the boxes 65, a portion of the gas makes a substantially180 turn to fiow upwardly through the boxes 65 and into the intakemanifold 51. Since the (bottom) inlet openings of the boxes 65 arehorizontal, the normal separating eifect due to the 180 turn is furtherenhanced by the effect of gravity on the entrained solids.

It should be noted that the boxes 65 span, and therefore lend structuralrigidity to, the air heater inlet duct 19 and the boundary of theconvection pass outlet 16A. Moreover, since the inlet openings to theboxes 65 extend substantially wholly across the air heater inlet duct 19and are evenly spaced across its width, representative portions of themain gas stream will be withdrawn for recirculation.

Referring to FIGS. 3 and 4, the alternate embodiment of the gas off-takesystem shown therein is arranged at the bottom of the upright convectionpass 16 and forms the outlet therefrom. Connected with the boundary wallof the convection pass is a horizontally extending air heater inlet duct79 in which the main gas stream is conveyed to an air heater (notshown). A plurality of gas off-take boxes or channel members 75 aresubstantially evenly spaced across the width of the unit at the junctionof the convection pass 16 and the air heater inlet duct 79. Each gasoff-take box 75 includes a pair of spaced, substantially parallel sideplates 76 of generally triangular shape joined at their upper streamends by a tapered end closure 77 to avoid excessive gas flow disturbanceand pressure drop. The two remaining ends 75A and 75B of each box 75 areopen to provide a flow channel therethrough for passage of the gaswithdrawn from the main gas stream for recirculation. The box inlet ends75A are disposed in a plane substantially perpendicular to the main gasflow stream. The outlet box ends 75B communicate with a laterallyextending intake manifold or cross duct 71 in which the gases withdrawnfrom the main stream are collected. Each end of the intake manifold 71connects with a recirculating fan inlet duct 72 through which thewithdrawn gases are conveyed to a recirculating fan (not shown).

In operation, as the solids-laden flue gas flows downwardly through theconvection pass 16 and executes the right angle bend into the air heaterinlet duct 79, a portion of the gas is constrained to make asubstantially turn to enter the off-take boxes 75 through their inletends 75A and flow therethrough into the intake manifold 71. The 180 turnof the gas entering the boxes 75 tends to separate entrained particulatematter, and a hopper 78 is provided immediately below the separationzone to collect the solids that settle out of the gas stream Byarranging the inlet openings to the boxes 75 in a plane having asubstantially horizontal component, the separation due to the 180 turnis enhanced by the effect of gravity acting on the entrained particles.It should be noted that the boxes 75 contribute significantly to thestructural rigidity of the ductwork in which they are enclosed sincethey span the convection pass 16, the air heater inlet duct 79 and theintake manifold 71.

What is claimed is:

1. A gas off-take system comprising first duct means having asubstantially rectangular cross-sectional flow area and forming apassageway confining the fiow of a stream of solids-laden gas, saidpassageway including a first portion, a second portion of greater widththan said first portion, and a flaring portion interconnecting saidfirst and second portions, second duct means disposed outside of saidpassageway transversely with respect to said first duct means forcollecting and confining the flow of gas withdrawn from said gas stream,and means for evenly distributing gas across the width of said secondportion of said first duct means including a plurality of gas off-takechannel elements spaced within said flaring portion at regular intervalsthereacross, said channel elements being oriented in diverging positionswith respect to each other in the direction of gas flow, each of saidchannel elements being in communication with said second duct means andhaving formed therein inlet opening means disposed within saidpassageway in a plane having a substantially horizontal component andbeing substantially perpendicular to the flow of said solids-laden gasin said passageway, whereby gas flowing through said passageway and intosaid opening means is required to take a substantial 180 turn to entersaid channel members.

2. A gas off-take system comprising first duct means having asubstantially rectangular cross-sectional flow area and forming apassageway confining the flow of a stream of solids-laden gas, saidpassageway including a first portion, a second portion of greater widththan said first portion, and a flaring portion interconnecting saidfirst and second portions, second duct means disposed outside of saidpassageway transversely with respect to said first duct means forcollecting and confining the flow of gas withdrawn from said gas stream,and means for evenly distributing gas across the width of said secondportion of said first duct means including a plurality of gas off-takechannel elements spaced within said flaring portion at regular intervalsthereacross and connected to opposite sides of said first duct means toafford structural rigidity thereto, said channel elements being orientedin diverging positions with respect to each other in the direction ofgas flow, each of said channel elements including a pair of spacedplates joined at their upstream ends by an end closure, said channelelements each having one end thereof in communication with said secondduct means and having another end thereof formed with an inlet openingdisposed Within said passageway in a plane having a substantialhorizontal component and being substantially perpendicular to the flowof said solids-laden gas in said passageway, whereby gas flowing throughsaid passageway and into said second duct means is required to take asubstantially 180 turn to enter said channel members.

3. In a vapor generating unit, walls defining a furnace chamber having aheating gas outlet, means for burning ash-bearing carbonaceous fuel insaid furnace whereby solids-laden heating gases are produced, wallsforming an upright heating gas pass communicating at its upper end withsaid heating gas outlet, heat exchange means arranged in said heatinggas pass, a combustion air heater, first duct means of substantiallyrectangular cross-section forming a passageway confining the flow ofsaid solidsladen gas from the lower outlet end of said heating gas passto said air heater, said passageway having at least one substantiallyright angle bend, and means for recirculating gas from said passagewayto said furnace chamber including a gas off-take system arranged withinsaid passageway at the bend thereof, said last named means includingsecond duct means communicating between said gas off-take system andsaid furnace chamber, and a gas recirculating fan interposed in saidsecond duct means, said gas off-take system comprising a plurality offluid off-take channel elements spaced at regular intervals across thewidth of said passageway at and in the plane of the bend thereof andconnected to opposite sides thereof to afford structural rigiditythereto, each of said channel elements being in fluid flow communicationwith said second duct means and having formed therein inlet openingmeans disposed within said passageway in a plane having a substantialhorizontal component and being substantially perpendicular to the flowof said solids-laden gas in said passageway, whereby gas flowing throughsaid passageway and into said second duct means is required to take asubstantially 180 turn to enter said channel members.

4. In a vapor generating unit, walls defining a furnace chamber having aheating gas outlet, means for burning ash-bearing carbonaceous fuel insaid furnace whereby solids-laden heating gases are produced, wallsforming an upright heating gas pass communicating at its upper end withsaid heating gas outlet, heat exchange means arranged in said heatinggas pass, a combustion air heater, first duct means of substantiallyrectangular cross section forming a passageway confining the flow ofsaid solids-laden gas from the lower outlet end of said heating gas passto said air heater, said passageway having at least one substantiallyright angle bend, and means for recirculating gas from said passagewayto said furnace chamber including a gas off-take system arranged withinsaid passageway at the bend thereof, said last named means includingsecond duct means communicating between said gas off-take system andsaid furnace chamber, and a gas recirculating fan interposed in saidsecond duct means, said gas off-take system comprising a plurality offluid off-take channel elements spaced at regular intervals across thewidth of said passageway at and in the plane of the bend thereof andconnected to opposite sides thereof to afford structural rigiditythereto, each of said channel elements being in fluid flow communicationwith said second duct means and having the downstream end thereof openfor communication with said passageway, said downstream ends beingdisposed in a plane having a substantial horizontal component and beingsubstantially perpendicular to the flow of said solids-laden gas in saidpassageway, whereby gas flowing through said passageway and into saidsecond duct means is required to take a turn to enter said channelmembers.

5. In a vapor generating unit, walls defining a furnace chamber having aheating gas outlet, means for burning ash-bearing carbonaceous fuel insaid furnace whereby solids-laden heating gases are produced, wallsforming an upright heating gas pass communicating at its upper end withsaid heating gas outlet, heat exchange means arranged in said heatinggas pass, a combustion air heater, first duct means of substantiallyrectangular cross section forming a passageway confining the flow ofsaid solidsladen gas from the lower outlet end of said heating gas passto said air heater, a portion of said passageway being substantiallynarrower than said air heater, said passageway having at least one bendportion and having an outwardly flaring portion disposed immediatelyupstream of said bend portion, and means for recirculating gas from saidpassageway to said furnace chamber including a gas Off-take systemarranged within said passageway at the bend portion thereof, said lastnamed means including second duet means communicating between said gasoff-take system and said furnace chamber, and a gas recirculating faninterposed in said second duct means, said gas offtake system comprisinga plurality of gas off-take channel elements spaced within said flaringportion at regular intervals thereacross and connected to opposite sidesof said first duct means to afford structural rigidity thereto, saidchannel elements being oriented in diverging positions with respect toeach other in the direction of gas flow to evenly distribute the gasleaving said flared portion, said channel elements each having one endthereof in communication with said second duct means and having anotherend thereof formed with an inlet opening disposed within said passagewayin a plane having a substantial horizontal component and beingsubstantial ly perpendicular to the flow of said solids-laden gas insaid passageway, whereby gas flowing through said passageway and intosaid duct means is required to take a substantially 180 turn to entersaid channel members.

6. In a vapor generating unit, walls defining a furnace chamber having aheating gas outlet, means for burning ash-bearing carbonaceous fuel insaid furnace whereby solids-laden heating gases are produced, wallsforming an upright heating gas pass communicating at its upper end withsaid heating gas outlet, a heat exchanger arranged in said gas pass, anair heater, wall structure forming a passageway confining the flow ofsaid solids-laden gas from the lower outlet end of said heating gas passto said air heater, said passageway having at least one substantiallyright angle bend, and means for recirculating gas from said passagewayto said furnace chamber including a gas off-take system arranged withinsaid passageway at the bend thereof, said gas recirculating meansfurther including a duct between said gas off-take system and saidfurnace chamber, and a gas recirculating fan in said duct, said ductincluding an intake manifold disposed outside of said passagewaytransversely with respect to said passageway, said gas off-take systemcomprising a plurality of gas off-take channel elements spaced atregular intervals transversely across said passageway at the bendthereof and connected to the wall structure thereof to lend structuralrigidity thereto, each of said channel elements being in fluid flowcommunication with the intake manifold of said duct and having formedtherein inlet opening means disposed within said passageway in a planehaving a substantial horizontal component and being substantiallyperpendicular to the flow of said solids-laden gas in said passageway,whereby that gas flowing through said passageway and withdrawn into 7said intake manifold is required to make a substantially 180 turn toenter said channel elements.

7. The combination according to claim 6 wherein said passageway iselongated in the direction of gas flow, each of said channel elementsincluding a pair of parallel plates which extend longitudinally and arespaced transversely relative to said passageway and which are joined attheir upstream ends by an end closure, the regular spacing of saidchannel elements providing distributed flow areas between them in saidpassageway for said solids-laden gas and further providing fiow areaswithin them for the reverse flow therethrough of gas withdrawn fromuniformly distributed locations across said passageway.

8. The combination according to claim 7 wherein said plates are oftriangular shape, each with a first edge 8 disposed on the upstream sideof said channel elements, a second edge on the downstream side of saidchannel elements within said passageway, and a third edge connected tosaid intake manifold.

References Cited by the Examiner UNITED STATES PATENTS KENNETH W.SPRAGUE, Primary Examiner.

1. A GAS OFF-TAKE SYSTEM COMPRISING FIRST DUCT MEANS HAVING ASUBSTANTIALLY RECTANGULAR CROSS-SECTIONAL FLOW AREA AND FORMING APASSAGEWAY CONFINING THE FLOW OF A STREAM OF SOLIDS-LADEN GAS, SAIDPASSAGEWAY INCLUDING A FIRST PORTION, A SECOND PORTION OF GREATER WIDTHTHAN SAID FIRST PORTION, AND A FLARING PORTION INTERCONNECTING SAIDFIRST AND SECOND PORTIONS, SECOND DUCT MEANS DISPOSED OUTSIDE OF SAIDPASSAGEWAY TRANSVERSELY WITH RESPECT TO SAID FIRST DUCT MEANS FORCOLLECTING AND CONFINING THE FLOW OF GAS WITHDRAWN FROM SAID GAS STREAM,AND MEANS FOR EVENLY DISTRIBUTING GAS ACROSS THE WIDTH OF SAID SECONDPORTION OF SAID FIRST DUCT MEANS INCLUDING A PLURALITY OF GAS OFF-TAKECHANNEL ELEMENTS SPACED WITHIN SAID FLARING PORTION AT REGULAR INTERVALSTHEREACROSS, SAID CHANNEL ELEMENTS BEING ORIENTED IN DIVERGING POSITIONSWITH RESPECT TO EACH OTHER IN THE DIRECTION OF GAS FLOW, EACH OF SAIDCHANNEL ELEMENTS BEING IN COMMUNICATION WITH SAID SECOND DUCT MEANS ANDHAVING FORMED THEREIN INLET OPENING MEANS DISPOSED WITHIN SAIDPASSAGEWAY IN A PLANE HAVING A SUBSTANTIALLY HORIZONTAL COMPONENT ANDBEING SUBSTANTIALLY PERPENDICULAR TO THE FLOW OF SAID SOLIDS-LADEN GASIN SAID PASSAGEWAY, WHEREBY GAS FLOWING THROUGH SAID PASSAGEWAY AND INTOSAID OPENING MEANS IS REQUIRED TO TAKE A SUBSTANTIAL 180* TURN TO ENTERSAID CHANNEL MEMBERS.